Indian Journal of ChemistryVol. 23A, December 1984, pp. 1057-1058

Iodometric Determination of MilligramAmounts of Some Carboxylic Acids by

Potentiometric Titration Method(Miss) A WADHWA & R M VERMA·

Department of Post-graduate Studies and Research inChemistry, University of Jabalpur, Jabalpur 482001

Received 20 March 1984; revised and accepted 18 July 1984

An iodometric method is described for the microdetermination ofsome carboxylic acids by potentiometric titration method usingplatinum-calomel electrode assembly. The proposed method can beapplied to the quantitative analysis of sample solutions containing0.01-0.1 meq. of the acids. The titration peaks are quite sharp evenwith 0.001 N of the titrand and the titrant solutions. An indirectmethod has also been worked out for determining certain acids towhich the direct method is not applicable.

Some of the difficulties associated with alkalimetrictitration of a dilute solution of a w.eak acid 1.2 areremoved when weak acids are potentiometricallytitrated with alkali using glass-calomel electrodecombination. But, due to the formation of a buffer 3

,

the potential jump near the equivalence point is smallwhich further decreases with increasing dilution of thetitrant, rendering the location of the end-pointdifficult.

An alternative method, based on iodometrictitration, was developed by Bruhns and Kolthoff4 forthe determination of weak acids. However, thismethod also was only partially successful and was notapplicable for analysis on micro scale. A visual micro-titrimetric procedure using solid potassium iodate andiodide has been reported 5 -7. The procedure is notapplicable for samples containing less than 0.025 meqof certain weak organic acids.

We report here a potentiometric method for theiodometric microdetermination of some weaklydissociated aliphatic acids. The method gives accurateresults for 0.01-0.1 meq of the tested acids. With higherfatty acids, which were dissolved in ethanol or DMF,the liberation of iodine on treatment with iodide-iodate reagent was too sluggish to be of any practicalapplication. In such cases an indirect method was used.

The following solutions were prepared using A Rgrade chemicals and conductivity water.

Potassium iodate solution (0.1 N) was prepared bydissolving calculated amount of the dried salt. A 5%(m/v) solution was also prepared. Sodium thiosulphate(0.1 N), oxalic acid (0.1N) and sodium hydroxide(0.1 N) solutions were prepared; these were diluted toobtain 0.01, 0.0025, 0.0012 anti 0.001 N solutions.Acid sample solutions (0.1N) were prepared and

standardized with sodium hydroxide and then suitablydiluted to prepare the test solutions. Owing to the poorsolubilities of heptanoic, octanoic, nonanoic anddecanoic acids in water, their solutions were preparedin 1:9, 1:2, I: 1 and 1:1 ethanol-water mixtures,respectively. The solution of dodecanoic acid wasprepared in ethanol and that of palmitic acid in DMF.

Procedure (I)A known aliquot (5-10 ml) of the test solution

containing 0.01-0.1 meq of the acid was treated with1g of potassium iodide and 5 ml of 5% potassiumiodate solution and contents were stirred by means of amagnetic stirrer. Platinum and calomel electrodes weredipped into the solution and thiosulphate solution(0.001-0.01 N) was gradually added through amicroburette. After each addition, the solution wasstirred and the potentiometric reading was recorded.In the case of nonanoic and decanoic acids, the rate ofreaction was found to be slow and a period of 2 minhad to be allowed after each addition for thecompletion of reaction. Later, the end-point waslocated graphically. The thiosulphate solution wasstandardized potentiometrically with a standardsolution of oxalic acid.Iml of 0.001 Nthiosulphate =0.045 mg of carboxylicfunction.The potential of- the indicator electrode (platinum)depends on iodide-iodine ratio, and because thischanges during the course of the titration, the platinumelectrode shows a potential change.

Procedure (II)

A sample solution (5-10 ml) containing 0.025-0.1meq of the acid was taken in a 100-mlErlenmeyer flaskand treated with 20 ml (0.005-0.01 N) potassium iodatesolution and Ig of potassium iodide. The flask wasimmersed into a boiling water-bath for 15-60 min (15min for dodecanoic acid and 60 min for palmitic acid).The flask was then cooled to room temperature andone ml of starch solution was added. Thiosulphatesolution was gradually added until the blue colourdisappeared. An excess of 2 N sulphuric acid was thenadded and the liberated iodine titrated against 0.005-0.01 N thiosulphate. The difference from the blankgave the amount of iodate used up by the acid.1 ml of 0.01 N thiosulphate =0.45 mg of carboxylfunction.

The proposed method has been applied for thedetermination of some carboxylic acids at four

1057

INDIAN J. CHEM., VOL. 23A, DECEMBER 1984

Table I-Determination of Some Carboxylic Acids byIodometric and Acid Base Potentiometric Titrations

Amount Iodometry Alkalimetrytaken(mg) Amount Av. St. Amount Av. St.

found" dev. dev. found" dev. dev.(mg) (:Yo) (yo> (mg) (yo> (yo>

13.018 13.0281.302 1.309

Heptanoic acid0.1 0.2 13.0280.5 0.2

0.1

Octanoic acid14.421 14.433 0.1 0.2 14.433 0.1 0.21.421 1.429 0.5 0.2

Nonanoic acid15.824 15.837 0.1 0.2 15.850 0.2 0.41.582 1.591 0.5 0.2

Decanoic acid17.226 17.240 0.1 0.2 17.240 0.1 0.21.723 1.732 Q.5 0.2

Malonic acid

10.406 10.414 0.1 0.2 10.423 0.2 0.31.041 1.047 0.6 0.2

Succinic acid11.809 11.829 0.2 0.3 11.838 0.2 0.41.181 1.188 0.5 0.3

Adipic acid14.614 14.626 0.1 0.2 14.638 0.2 0.31.461 1.469 0.5 0.2

•Average of six determinations

different concentrations and the results have beencompared with those obtained with acid-basepotentiometry (Table I). These comparative resultsshow that the iodometric method is satisfactory fordetermining the tested acids in the concentration range0.001-0.0 I N (average deviation 0.1-0.5%). Thepotential change ;n the vicinity of the equivalencepoint is 80-100 mV/0.1 ml of the titrant. In the acid-base potentiometry this potential change reduces to

0.4

30-40 mV with 0.0025-0.01 N solutions; with moredilute solutions the potential change further reduces to10·15 mV/0.1 ml of the titrant added. Besides, a seriesof small and equal jumps are noted in the vicinity of theequivalence point; consequently, the location of theend-point is not possible below 0.0025 N concentrationlevel. The potentiometric titration based on iodometrygives sharp titration peaks even with 0.001 N solutions.

An additional advantage ofiodometric method is itsapplicability for the analysis of carboxylic acids whichcontain another component capable of reacting withalkali. For example, in determining carboxylic acids inlac, the alkalimetry cannot yield accurate results owingto the saponification of the lac resin. Dodecanoic andpalmitic acids have very poor solubilities in water;hence, their solutions were prepared in ethanol andDMF respectively. In these solvents, the directiodometry (procedure I) was not successful owing toincomplete J,iberation of iodine. Hence, an indirectprocedure (procedure II) based on the measurement ofthe iodate consumed rather than that of iodineliberated, has been applied. The results show that thealkalimetry in the case of these acids is unsuitable(error 0.3-11.2%). The indirect iodometry, however,gives an average error in the range of 0.2-0.8% withsamples containing 0.025-0.1 meq of the acid.

One of us (A.W.) is thankful to the CSIR, New Delhifor the award of a senior research fellowship.

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Wiley, New York) 1964,402,485.2 Steyermark A, Quantitative organic microanalysis (Academic

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York) 1957,276,277.5 Nema S N & Verma R M, Talanta, 25 (1978) 400.

6 Saxena R, Pateria M G, SoniG P & Verma R M, Microchem J, 26(19tH) 334.

7 Nema S N & Verma R M, Analyst, 104 (1979) 691.